In this paper, a new reconfigurable microstrip structure is presented where line impedance can be actively controlled to induce phase shift on a transmission-line signal. Micromachining is used to fabricate thin-film copper membranes in the ground plane below the transmission line. Actuation of these membranes enables control of transmission-line to ground-plane spacing and, thus, the line capacitance. The performance of this reconfigurable microstrip transmission line as a phase shifter is investigated between 5-32 GHz for a variety of membrane geometries. A phase shift of up to -32.1degrees at 15.00 GHz is achieved by actuating an array of five 4.3-mm-diameter membranes beneath a 30-Omega microstrip line. A single 10.0-mm-diameter membrane achieved a phase shift of -25.57degrees at 15.05 GHz on a 5642 line, while a 10.4-mm membrane achieved a phase shift of -55.5degrees at 14.25 GHz on a 30-Omega line. Both single and multiple series ground-plane membranes are possible, and they can be activated with discrete or continuous control signals, individually or together. Discrete and continuous phased array beam steering are, therefore, feasible.